1. Field of the Invention
This invention relates to a process of directly producing a silicon ribbon with a p-n junction formed therein through super-rapid cooling of the melt of silicon material. More particularly, this invention relates to a process of producing a silicon ribber with a p-n junction suitably available for solar cells.
2. Description of the Prior Art
The solar cells, or more precisely photovoltaic cells, have been drawing much attention as one of promising future energy sources of most desirable type, because the solar cells convert sunlight directly into electric energy without using any moving parts, and the solar cells can be operated with easy maintenance and can be readily adapted to remote supervision without any attending residence personnel at site. Besides, the solar cells are formed in module structure, which is suitable for mass production, and their conversion efficiency for generating electricity is substantially constant regardless of the size of power plants composed thereof. The idea of solar cells was proposed by Mr. Chapin in 1954 and theorized next year by Mr. Prince. Numerous experiments on solar cells using silicon (Si) and cadmium sulphide (CdS) as fundamental materials thereof have been reported since then. The energy crisis of recent years has accelerated the studies of the solar cells, and a number of new semiconductor materials other than the above-mentioned fundamental materials have been developed and their viability has been investigated. Examples of such new materials are gallium arsenide (GaAs), the films of indium phosphide (InP), amorphous silicon, cadmium telluride (CdTe), and suitable mixtures thereof. Nevertheless, most researchers of the solar cells generally consider that silicon would be finally selected for practical use due to the following reasons: namely, although silicon solar cells are expensive at the present as the fundamental materials for power plants, considerable cost reduction is possible if a large amount of silicon solar cells are produced on truly mass production basis for residential power sources and commercial power sources; the physical properties of the silicon solar cells have been clarified in considerable detail; and the resource of silicon is abundant.
Conventionally, silicon solar cells have been made by the Czochralski process, the horizontal pull process, or vertical pull process. The conventional processes have a shortcoming in that their production speed is very low. For instance, the horizontal pull process, which is generally considered to be the fastest, produces silicon ribbon at a rate of 15 to 20 cm per minute. If it is assumed that silicon solar cells were used to generate that much power which corresponds to an average growth increment (e.g., about 5%) of the total power consumption in Japan, and if the silicon solar cells are assumed to have a conversion efficiency of 10% and be in the form of 5 cm wide ribbon wafers produced by the horizontal pull process, about 10,000 units of silicon solar cell producing equipment would become necessary. Practicability of such a large number of silicon solar cell producing units is fairly low, so that it can be concluded that, unless the speed of producing the silicon ribbon is improved, the silicon solar cells cannot meet even the average growth increment of the power consumption. Under such conditions, the silicon solar cells hardly claim the position of a viable alternative energy source.
Previously, the inventors succeded in producing silicon ribbons at a rate of about 70 m per second by a super-rapid melt cooling method, as disclosed in U.S. patent application Ser. No. 961,047, now abandoned in favor of U.S. Pat. No. 4,363,769.
The above-mentioned super-rapid melt cooling method comprises steps of placing raw silicon material in a heat-resisting tube such as a quartz tube with a nozzle, melting the silicon material by an electric furnace, ejecting the melt of the silicon material through the nozzle, i.e., an orifice at the lower end of the heat-resisting tube by applying a high pressure argon gas to the heat-resisting tube onto the circumferential surface of a disk roller revolving at a high speed or twin-roller type cooling substrate; and forming a silicon ribbon by super-rapid cooling of the thus ejected melt. The feature of the super-rapid melt cooling method of producing the silicon ribbon is that the speed of producing the silicon ribbon is more than one thousand times faster than that of the conventional processes such as the horizontal pull process and that the growth process of silicon ribbon is very simple. Besides, the silicon ribbon produced by the super-rapid melt cooling method has a thickness of about 20 to 100 .mu.m which is best suited to solar cells, and the crystal structure of the silicon ribbon wafer is of columnar crystals with crystal grains extending therethrough at right angles to the ribbon surface, so that the silicon ribbon is free from unstableness experienced in amorphous semiconductor solar cells. Thus, the super-rapid melt cooling method has attracted much interest as one of the most promising candidates for producing the solar cells in the future.
Although the super-rapid melt cooling method produces the silicon ribbon at a high speed such as about 20 m per second as mentioned above, in order to make silicon solar cells, barriers or junctions must be formed in the silicon ribbon wafer in the form of a p-n junction or a Schottky barrier. Unless such junction or barrier is formed quickly, overall high speed efficiency cannot be achieved in fabricating the silicon solar cells to minimize the production cost thereof. Besides, there is a need for further improvement of the super-rapid melt cooling method itself, because the method is not free from considerable unevenness in the surface conditions, the width non-uniformity, and the edge non-linearity of the silicon ribbon wafer produced thereby due to changes in the processing conditions.
Heretofore, the junctions or barriers such as the p-n junctions or Schottky barriers have been formed by the so-called CVD (Chemical Vapour Deposition) method or by spread-diffusion method after the silicon ribbon wafers are produced. For instance, in the spread-diffusion method of the prior art, silicon tetrachloride (SiCl.sub.4) dissolved in acetic acid (CH.sub.3 COOH) is spread on a silicon ribbon wafer, which is prepared by the melt super-rapid cooling method, and the silicon ribbon wafer is then reheated in an argon (Ar) gas atmosphere at about 900.degree. C. for about 30 minutes to one hour. The conventional methods of forming the junctions or barriers in the silicon ribbon wafers have a shortcoming in that their production speed is very low since said reheating is necessary.
More particularly, the speed at which the silicon ribbons are made, such fast making of the silicon ribbon does not mean much in the production of silicon solar cells unless the junctions such as the p-n junctions are formed quickly, possibly simultaneously with the making of the silicon ribbons. For instance, if much time is required in forming the p-n junctions such as by reheating, the production of the silicon solar cells is restricted at the stage of forming the p-n junctions, and the merit of the high-speed production of the silicon ribbon is diluted. Apart from the production speed, smooth surface without undulations or unevenness is an absolute requirement for silicon ribbon wafers to be used in silicon solar cells, because the smooth surface of the silicon ribbon is necessary to produce uniform junctions and to ensure stable bonding of electrodes thereto. In addition, to make solar cells, uniform widths and linear edges of the silicon ribbon are desirable.